Matt,
I'm not sure it's quite as simple as you seem to be suggesting. After reading 119.1(e)(2), my bet is a lot of pilots would think that they don't have to do anything to be "set up" to do some local sightseeing flights.
Sightseeing is one of those things that you can do as a commercial pilot without going the 135 route. But there is a regulatory catch: you need to have a Part 135-style drug program in place.
Sightseeing flights within 25 NM which start and end at the same airport are exempt from operator certificate requirements under 119.1(e)(2). But 135.1(a)(5) lists a group of Part 135 rules that apply to these flights anyway. Those rules, contained in 135.249, 135.251, 135.253, 135.255, and 135.353 are the drug and drug-testing rules.
What this means is that you may do sightseeing flights under Part 91 with only a commercial pilot certificate and without a Part 135 operator certificate. But (and it's a BIG but), you must have an official Part 135 drug program in place. Ends up being cost-prohibitive for some.
(There are also likely to be insurance issues. If you're renting, he FBO policy might not cover commercial operations other than flight training. If you're using your own, chances are that unless you have a commercial policy, these trips are not covered.